Layered photonic crystals

ABSTRACT

A three dimensional photonic crystal and layer-by-layer processes of fabricating the photonic crystal. A substrate is exposed to a plurality of first microspheres made of a first material, the first material being of a type that will bond to the templated substrate and form a self-passivated layer of first microspheres to produce a first layer. The first layer is exposed to a plurality of second microspheres made of a second material, the second material being of a type that will bond to the first layer and form a self-passivated layer of second microspheres. This layering of alternating first and second microspheres can be repeated as desired to build a three dimensional photonic crystal of desired geometry. Charged polymers such as polyelectrolyte coatings can be used to create the bonds.

CROSS REFERENCE TO RELATED DOCUMENTS

This application is a continuation in part of U.S. patent applicationSer. No. 10/210,010, filed Jul. 31, 2002, now U.S. Pat. No. 6,782,868entitled “Layer-By-Layer Assembly of Photonic Crystals” to John SouthLewis, III, et al which is hereby incorporated by reference.

STATEMENT OF U.S. GOVERNMENT RIGHTS UNDER 35USC 202(C)(6)

The U.S. government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of contractnumber N66001-01-1-8938 awarded by the United States Defense AdvancedResearch Project Agency.

FIELD OF THE INVENTION

This invention relates generally to the field of photonic crystals. Moreparticularly, this invention relates to a photonic crystal structure anda method for layer-by-layer fabrication of such crystal structure.

BACKGROUND

Photonic crystals are being actively pursued as components in opticalnetworks, such as wavelength-division multiplexing applications.Examples of potential applications are as filters, mirrors, waveguides,and prisms. Added functionality could allow the crystals to be used inother applications such as frequency-tunable filters, optical switches,chemical and biological recognition systems, as well as other potentialapplications.

Three dimensional (3-D) photonic crystals have been made using a numberof approaches. One common approach uses a colloidal technique thatallows a distribution of spheres (e.g. microspheres—typically spheresapproximately in the range of 90 nm to several microns in diameter) tosettle out of solution into a bulk 3-D crystal. A similar approach usesthe surface tension of a moving liquid/gas interface, created by eitherby pulling a substrate out of a liquid or by evaporating the liquid, tocreate a 3-D crystal made of up microspheres. Both techniques result ina close-packed structure of identical spheres. More complex structuresare possible if differently sized spheres are used, but there is verylittle external control over the crystallization process and theresulting structure. The spheres can be made with a number of differentmaterials, with polystyrene a common example, and the components areuniform in size and composition. Crystals fabricated using thistechnique are mechanically unstable unless a matrix such as a polymermatrix is used between the spheres to mechanically reinforce thestructure.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

The present invention relates generally to photonic crystals. Objects,advantages and features of the invention will become apparent to thoseskilled in the art upon consideration of the following detaileddescription of the invention.

In general terms, without any intention of limiting the invention, thepresent invention, in certain embodiments, relates to a technique whichcan be used to fabricate photonic crystals in a controlled,layer-by-layer manner. This allows control over parameters not possiblewith traditional colloidal techniques and permits novel crystalstructures to be created.

A method of fabricating a photonic crystal, consistent with certainembodiments involves providing a substrate; exposing the substrate to aplurality of first microspheres made of a first material, the firstmaterial being of a type that will bond to the substrate and form aself-passivated layer of first microspheres to produce a first layer;and exposing the first layer to a plurality of second microspheres madeof a second material, the second material being of a type that will bondto the first layer and form a self-passivated second layer of secondmicrospheres.

Another method of fabricating a photonic crystal, consistent withcertain embodiments involves a) providing a substrate; b) exposing thesubstrate to a plurality of first microspheres made of a first material,the first material being of a type that will bond to the substrate andform a self-passivated layer of first microspheres to produce a layer ofmicrospheres; c) modifying the first layer of microspheres to permit thefirst layer of microspheres to bond with other microspheres to therebyproduce a bondable layer; and d) exposing the bondable layer to aplurality of second microspheres to form a second layer of microspheres.

A photonic crystal structure, consistent with certain embodiments has asubstrate processed to bond preferentially to a first material inselected areas with a first layer of first microspheres, the first layerbeing one microsphere deep, the first microspheres comprising the firstmaterial and bonded to the selected areas of the substrate. A secondlayer of second microspheres one microsphere deep is bonded to the firstlayer of microspheres.

Another method of fabricating a photonic crystal, consistent withcertain embodiments involves providing a substrate; bonding a singlelayer of microspheres one microsphere deep to the substrate to form afirst layer; and bonding a single layer of microspheres one microspheredeep to the first layer to form a second layer.

Another method of fabricating a photonic crystal, consistent withcertain embodiments involves providing a templated substrate having afirst charge; and exposing the templated substrate to a plurality offirst microspheres having a polyelectrolyte coating carrying a secondcharge, the second charge being opposite the first charge so that theplurality of first microspheres will bond to the templated substrate andform a self-passivated layer of first microspheres to produce a firstlayer.

Another method of fabricating a photonic crystal, consistent withcertain embodiments involves: a) providing a templated substrate; b)exposing the templated substrate to a plurality of first microspheresmade of a first material, the first material being of a type that willbond to the templated substrate and form a self-passivated layer offirst microspheres to produce a layer of microspheres; c) modifying thefirst layer of microspheres to permit the first layer of microspheres tobond with other microspheres to thereby produce a bondable layer bycoating the first microspheres with a polyelectrolyte film having afirst charge; and d) exposing the bondable layer to a plurality ofsecond microspheres having charge opposite the first charge to form asecond layer of microspheres.

A photonic crystal structure, consistent with certain embodiments has atemplated substrate processed to bond preferentially to a first materialin selected areas. A first layer of first microspheres, the first layerbeing one microsphere deep, (the first microspheres comprising the firstmaterial) is bonded to the selected areas of the templated substrate. Acharged polymer coating is on the first microspheres.

A method of fabricating a photonic crystal, consistent with certainembodiments involves providing a templated substrate; bonding a singlelayer of charged polymer coated microspheres one microsphere deep to thetemplated substrate to form a first layer; and bonding a single layer ofcharged polymer coated microspheres one microsphere deep to the firstlayer to form a second layer.

A method of fabricating a photonic crystal consistent with certainembodiments involves bonding a single layer of charged polymer coatedmicrospheres one microsphere deep to a substrate to form a first layer;and bonding a single layer of charged polymer coated microspheres onemicrosphere deep to the first layer to form a second layer.

A three dimensional photonic crystal and layer-by-layer processes offabricating the photonic crystal consistent with certain embodiments hasa substrate that is exposed to a plurality of first microspheres made ofa first material, the first material being of a type that will bond tothe templated substrate and form a self-passivated layer of firstmicrospheres to produce a first layer. The first layer is exposed to aplurality of second microspheres made of a second material, the secondmaterial being of a type that will bond to the first layer and form aself-passivated layer of second microspheres. This layering ofalternating first and second microspheres can be repeated as desired tobuild a three dimensional photonic crystal of desired geometry. Chargedpolymers such as polyelectrolyte coatings can be used to create thebonds.

The above summaries are intended to illustrate exemplary embodiments ofthe invention, which will be best understood in conjunction with thedetailed description to follow, and are not intended to limit the scopeof the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself however, bothas to organization and method of operation, together with objects andadvantages thereof, may be best understood by reference to the followingdetailed description of the invention, which describes certain exemplaryembodiments of the invention, taken in conjunction with the accompanyingdrawings in which:

FIG. 1 illustrates an exemplary substrate consistent with certainembodiments of the present invention.

FIG. 2 illustrates a single layer of microspheres bonded to thesubstrate 100 in a manner consistent with certain embodiments of thepresent invention to create a single layer photonic crystal.

FIG. 3 illustrates a two layer photonic crystalline structure ofmicrospheres fabricated on the substrate 100 in a manner consistent withcertain embodiments of the present invention.

FIG. 4 illustrates a three layer photonic crystalline structure ofmicrospheres fabricated on the substrate 100 in a manner consistent withcertain embodiments of the present invention.

FIG. 5 is a perspective view of a substrate having etched invertedpyramids in an upper surface thereof consistent with certain embodimentsof the present invention.

FIG. 6 is a side cutaway view of an etched inverted pyramid along linesA—A of FIG. 5 consistent with certain embodiments of the presentinvention.

FIG. 7 is a flow chart depicting a process for layer-by-layerfabrication of a photonic crystalline structure consistent with certainembodiments of the present invention.

FIG. 8 is a flow chart depicting an alternative process forlayer-by-layer fabrication of a photonic crystalline structureconsistent with certain embodiments of the present invention.

DETAILED DESCRIPTION

While this invention is susceptible of embodiment in many differentforms, there is shown in the drawings and will herein be described indetail specific embodiments, with the understanding that the presentdisclosure is to be considered as an example of the principles of theinvention and not intended to limit the invention to the specificembodiments shown and described. In the description below, likereference numerals are used to describe the same, similar orcorresponding parts in the several views of the drawings.

The current invention, in its many embodiments, offers severalapproaches that could be used to build a photonic crystalline structurein a layer-by-layer fashion. This has the potential of improving thequality of the resultant crystals, but also offers the ability toengineer the structure of the crystal using spheres with varyingproperties, and substrates patterned to achieve varying effects.

One method, consistent with certain embodiments of the presentinvention, uses a type of biological recognition as a method forfabricating multi-layer structures in a selective manner. The detailedexample provided below focuses on using biotin- and streptavidin-coatedmicrospheres as the basis for the photonic crystals. Streptavidin is aprotein while biotin is a vitamin. These are well-known molecules whichbond with one another in a biological bond with a strength approachingthat of a covalent bond, but the spheres do not tend to bond tothemselves (aggregate). Other types of biological recognition such asprotein to protein, DNA to DNA, etc. will be described later. Thesecoated microspheres are readily available commercially, and are commonlyused in the process of fluorescence microscopy to indicate the presenceof other proteins.

In accordance with certain embodiments of the present invention, alayer-by-layer approach is taken to fabrication of a photonic crystal.Many variations of techniques consistent with the current teachings arepossible without departing from the invention. One embodiment using theabove-mentioned biotin- and streptavidin coated spheres is illustratedin FIGS. 1-4 wherein a three layer photonic crystal is constructed usingsuch microspheres. In this exemplary embodiment, alternating layers areconstructed using biological bonds (i.e., protein bonds) to secure thelayers together and to a substrate. Turning first to FIG. 1, a substrate100 can be “templated” or “patterned” to capture microspheres of thedesired size in a desired arrangement. In this simple example, thetemplating is done by starting with a metal coated substrate (e.g., agold plated substrate) and etching away metal to form regularly spaceddisks of metal such as 104 using any suitable etching technique.

Suitable substrates can be devised of many types of materials including,but not limited to, glass, quartz, silicon, germanium, gallium arsenide,photoresist, ceramic, epoxies, polymers, plastics, and metals: The mainconsideration, from the perspective of actual layer-by-layer fabricationof the photonic crystal, is that the substrate have a surface forbonding of the microspheres which is very smooth in comparison with thesize of the microspheres.

The disks 104 are sized to accommodate a single layer of microsphereswith a single microsphere centered on each disk 104. Thus, if 2.0 microndiameter microspheres are used, the disks would be spaced 2.0 micronsapart center to center. The templating technique used, of course, shouldbe matched to the resolution required to achieve the precision needed toaccommodate a particular size of microspheres. The substrate can be ofany suitable material such as previously described.

For this exemplary embodiment, the metal disks remaining from theetching are then treated to make them suitable for bonding with thedesired first layer of microspheres. In the present exemplaryembodiment, the first layer can be made of streptavidin-coatedmicrosphere beads (available from Bangs Laboratories, Inc., 9025Technology Drive, Fishers, Ind. 46038 and other commercial sources).Thus, in the present example, to permit bonding with thestreptavidin-coated microspheres the disks 104 are first plated withgold and then biotinylated using any known process for biotinylatinggold. In other embodiments, any mechanism for biotinylating a specifiedregion of the substrate can be used to create regions of preferentialbonding for the streptavidin-coated microspheres. Since streptavidinreadily forms a biological bond with biotin, but will not bond withitself, a first layer of microspheres can be applied to the substrate bysuspending the microspheres in a colloidal solution or slurry andbringing the solution or slurry into contact with the templatedsubstrate 100. This produces a single layer of streptavidinmicrospheres. Since streptavidin spheres do not readily bond tothemselves, this layer is referred to herein as “self-passivated”. Thatis, once the single layer is formed, other streptavidin spheres do nottend to bond to the single layer. (Other equivalent techniques couldhypothetically involve applying microspheres that do not explicitlyself-passivate, if excess microspheres can then be removed to leave asingle layer. Such equivalents to self passivated layers arecontemplated by the present invention.)

Many variables are possible in this stage, including but not limited to,the temperature, pH, slurry concentration, agitation, composition of theliquid component of the slurry and contact time. These variables can beoptimized by experimentation to achieve a desired coating of a singlelayer of microspheres such as illustrated in FIG. 2. In this figure, thestreptavidin-coated microspheres 110 preferentially situate themselvesin a single layer with the disks 104 and bond to the disks with a verystrong protein bond. That is, a layer of microspheres which is onemicrosphere deep is deposited on the substrate and bonds therewith.

Once a complete layer of the streptavidin-coated microspheres 110 isbonded to the array of disks 104, the remaining streptavidin-coatedmicrospheres present in the slurry or solution have nowhere to bond andcan be readily rinsed away, for example with a water rinse (andpotentially reused). At this stage, a single layer photonic crystallinestructure 120 is formed and is precisely a single layer of microspheresin thickness above the substrate 100.

In order to continue building a thicker photonic crystal, a second layer(again one microsphere deep) can be added. In this example, a secondlayer of streptavidin-coated microspheres will not bond to the firstlayer of streptavidin-coated microspheres 110. A second layer, however,can be constructed using a complementary type of microsphere that willform a protein bond with the streptavidin-coated microspheres 110 asillustrated by the two layer photonic crystal illustrated as 130 in FIG.3. In order to apply a second layer of microspheres, biotin coatedmicrospherical beads 134 of the same size as the streptavidin-coatedmicrospheres 110 are used (also available from Bangs Laboratories, Inc.,9025 Technology Drive, Fishers, Ind. 46038 and other commercialsources). The biotin coated microspheres 134 are again placed in asuitable slurry and the single layer photonic crystal 120 is exposed tothe slurry (again with variables such as the temperature, pH, slurryconcentration, agitation, composition of the liquid component of theslurry and contact time optimized by experimentation to achieve adesired coating of a single layer of biotin coated microspheres 134bonded to the streptavidin-coated microspheres 110). Under suitableconditions, the second layer will form a strong biological bond with thefirst layer in a single layer mechanical arrangement as illustrated. Thepacking of the second layer is provided by the topographic features ofthe first layer of spheres (110), wherein multiple bonding surfaces(four surfaces in the example shown in FIG. 3) are available to thesecond layer at the points between spheres, providing the most stablearrangement. In addition to a larger number of bonding surfaces, thereduced amount of Brownian fluid forces on the microsphere once it islocated in a recessed area will assist in stabilizing the second layerin an arrangement consistent with that of the first layer. Thus thepacking of the second layer 134 is determined by the pattern provided bythe first layer 110, such that a variety of three-dimensional structurescan be obtained by modifications to the arrangement of microspheres inthe first layer. As with the first layer, the second layerself-passivates when no more spaces are available on the first layer ofstreptavidin-coated microspheres 110 for bonding by the biotin coatedmicrospheres 134. The remaining biotin coated microspheres in the slurrycan then be rinsed away. Thus, the second layer is formed to produce atwo layer photonic crystal structure 130.

A third layer can be fabricated using a similar technique to that of thefirst layer. To form the third layer, streptavidin-coated microspheresare again used since they will bond with the biotin coated microspheres134 of the second layer with a protein bond. Since streptavidin readilyforms a protein bond with biotin, but will not bond with itself, a thirdlayer of microspheres can again be applied to the substrate by placingthe streptavidin microspheres in a liquid solution or slurry andbringing the slurry into contact with the two layer photonic crystal130. Again, many variables are possible in this step, including but notlimited to, the temperature, pH, slurry concentration, agitation,composition of the liquid component of the slurry and contact time.These variables can be optimized by experimentation to achieve a desiredcoating of a single layer of microspheres 140 (one microsphere deep)bonded to the layer of microspheres 134 such as illustrated in FIG. 4.In this figure, the streptavidin-coated microspheres 140 preferentiallysituate themselves in a single layer with the microspheres 134 and bondto the microspheres 134 with a very strong protein bond. Once a completelayer of the streptavidin-coated microspheres 140 is bonded to the layerof microspheres 134, the remaining streptavidin-coated microspherespresent in the slurry have nowhere to bond and can be readily rinsedaway and potentially reused. At this stage, a three layer photoniccrystal 150 is formed.

This process can be repeated sequentially building layer after layer ofalternating streptavidin-coated microspheres and biotin-coatedmicrospheres to form a desired sized and shaped crystalline structure.

Initial experiments have been conducted to fabricate a photonic crystalusing the above-described method. Results show that the two types ofmicrospheres do in fact preferentially bond with one another, and that atwo-layer structure can be deposited on a glass slide serving as anunpatterned (untemplated) substrate. In these experiments, this wasaccomplished by bonding streptavidin coated spheres to a glass slide tocreate an unpatterned irregular monolayer by placing a drop of watercontaining 1% solids (1 mg of microspheres per 100 mL of water) in achamber on a glass slide created by bordering a section of the slidewith tape and placing a cover over the section. The streptavidin spheresnaturally bond to glass to create an irregular monolayer ofstreptavidin. After flushing away excess streptavidin-coated sphereswith water, this monolayer was then exposed to a 1% solids solution(again 1 mg of solids per 100 milliliters of water) of biotin-coatedmicrospheres at room temperature for 30 seconds to five minutes. In thisexample, the substrate and thus the layers are not patterned, so theresult was a two-layer random arrangement of microspheres. Experimentalresults show that higher concentrations of the spheres in the solution,and longer waiting time for the solution to settle (without agitation)resulted in higher concentration of deposited and bonded spheres. Theexperiments suggest that use of a patterned or templated substrate asdescribed above would permit engineering of the mechanical structure inthe layer-by-layer fashion described above.

Selection of an appropriate substrate material depends upon the type andsize of microsphere and type of bonding to be used to assemble thelayers, as well as the patterning or templating to be used.

Patterning the substrate to control the first layer of microspheres maypresent a technical challenge, depending on the size of the microspheresto be used, but substrate templating has been demonstrated in theliterature for colloidal assembly approaches, and templating thesubstrate with respect to the present method should not present any newchallenges. The continuous improvements in the ability to patternsubstrates are expected to permit smaller and smaller microspheres to beused. The particular approach used to pattern or template the substratedepends, in part, on the type of chemistry used for bonding themicrospheres together. It may also be possible to produce multiplelayers using the technique described without use of a templatedsubstrate, although templating is preferred.

Many methods could potentially be used to template the substrate.Generally, methods of templating the substrate that can potentially beused include chemical patterning, physical patterning, or a combinationof the two.

Physical patterning of the topography of the substrate can alsopotentially be used, for example, as illustrated in FIGS. 5 and 6. Oneapproach to physical patterning would use anisotropic etching ofperiodic inverted pyramids such as 204 and 206 into the substrate 212 atan upper surface 216 thereof. Using four sided pyramids, such as pyramid204 this would give the microspheres four bonding sites at each of theetched surfaces 220, 222, 224 and 226 in the pyramid rather than justone on a plane surface such as the upper surface 216, and it is expectedthat the spheres would preferentially settle and bond inside thepyramids such as 204 and 206. Inverted pyramidal shapes having three ormore sides can be used without departing from the present invention.Moreover, pyramids are only one shape that could be etched or otherwisemilled into a substrate to provide multiple bonding sites that sphereswould likely consider preferential to a single surface site. Othertechniques, such as isotropic etching; mechanical machining; molding;micro-contact printing; UV, deep UV, x-ray, electron beam, or ion beamlithography; ion beam milling; holographic patterning; two-photonpolymerization; or some other suitable technique can also equivalentlybe used, in appropriate combinations if necessary, to pattern a surfaceof the substrate.

Topographical patterning generally uses a substrate with uniformchemical composition, but this should not be considered limiting sincemany exceptions can be conceived. The topographical patterns providepreferential bonding sites for the beads based on physical topography.Examples are rounded or pyramidal recesses in the substrate surface. Therecesses give larger areas for bonding or additional surfaces forformation of bonds between the microspheres and the substrate.

Chemical selectivity can be used to template the substrate by creatinglocal regions of the substrate where the microspheres tend to bond withthe substrate by use of a chemical with appropriate bonding properties.The remainder of the substrate can be made of a different material. Oneexample would be to use a background material which does not bond withstreptavidin-coated beads, such as photoresist. Adjacent to thephotoresist, regions are patterned which will form bonds withstreptavidin coated beads. Examples of such regions could includebiotinylated surfaces such as gold, or other surfaces such as glass,silicon, silicon dioxide, silicon nitride, etc. Chemical selectivity inthis context also includes bonding due to electrostatic or ionicattraction. Many other examples may occur to those skilled in the artupon consideration of this teaching.

A hybrid of chemical selectivity and topographical patterning can alsobe used. In one example of such a hybrid approach, streptavidin-coatedbeads adhere to such surfaces as glass, silicon, silicon dioxide,silicon nitride, and gold. They do not adhere well to certain types ofphotoresist. Patterning holes in the photoresist (via traditional UV orelectron beam lithography) results in a structure that is chemicallyselective, and the height of the residual photoresist provides a degreeof physical templating as well. Either chemical selectivity or physicaltemplating of the substrate can potentially, but not necessarily, beadaptable to include intentional defects into the crystal structure ifdesired.

Thus, methods consistent with embodiments of the present invention canuse forming a geometric pattern in the substrate material to createpreferential bonding regions on the substrate, forming three-dimensionaltopography on a surface of the substrate to create preferential bondingregions within the topography, forming inverted pyramid shaped recesseson a surface of the substrate material to create preferential bondingregions within the inverted pyramids or chemically treating thesubstrate to create preferential bonding regions on the substrate. Inaddition, combinations of these techniques can be used such as creatingpreferential bonding regions on the substrate by a combination ofchemical and topographical patterning. Other techniques may occur tothose skilled in the art upon consideration of the present invention.

The above process, as depicted in FIGS. 1-4, is outlined in flow chart300 of FIG. 7 starting at 304. A suitable substrate is prepared orobtained at 308 so that spheres of a particular type (type A in thisexample) will bond to the substrate in a manner dictated by thepatterning or templating of the substrate. It is hypothesized that asuitable bond can result from many types of bonding and attractionphenomenon such as covalent bonding, electrostatic attraction, metallicbonding, hydrogen bonding (electrostatic attraction between anelectronegative atom and a hydrogen atom that is bonded covalently to asecond electronegative atom), Van der Waals forces,hydrophobic/hydrophilic attractions (hydrophobic attractions causenon-polar groups such as hydrocarbon chains to associate with oneanother in an aqueous environment), biological recognition such asprotein-protein/protein-ligand complexes (e.g., antigen-antibody), DNAor RNA hybridization or ligand-receptor (e.g., enzyme-substrate)(biological recognition generally results from a three dimensionalstructure that allows multiple weak forces between molecules), or somecombination of the above forces, or any other suitable bond. The spheresof type A are then brought into contact with the prepared surface of thesubstrate at 312 by, for example, immersing the substrate in a solutioncontaining the spheres or exposing the prepared surface to such asolution or slurry containing the microspheres. At this point, dependingupon the materials and type of bonding, agitation, heating or otheractions may be taken to enhance the speed or consistency of the bondingof the type A spheres to the substrate. Once a self-passivated (orotherwise equivalently self-limiting) single layer of spheres havebonded to the surface of the substrate, the excess type A spheres can beremoved, for example by rinsing at 316.

If multiple layers of spheres are desired at 320, the process proceedsto 324 where a second type of spheres (type B) is brought into contactwith the first layer of type A spheres bonded to the substrate. Type Bspheres are spheres that bond to type A spheres but not to themselves.Again, depending upon the materials and type of bonding, agitation,sonication, heating or other actions may be taken to enhance the speedor consistency of the bonding of the type B spheres to the first layerof type A spheres. Once a self-passivated single layer of type B sphereshave bonded to the layer of type A spheres on the substrate, the excesstype B spheres can be removed, for example by rinsing at 328. If onlytwo layers are desired at 332, the process is halted at 336. However, ifadditional layers are desired, one merely repeats 312 and 316 (withpossible process adjustments to account for bonding between the twotypes of spheres rather than spheres to substrate).

This process can be repeated until a desired odd number of layers isreached at 320 or even number of layers is reached at 332, at whichpoint the process can be halted at 336. Those skilled in the art willappreciate that the present exemplary embodiment uses two differentcomplementary types of spheres, but this should not be consideredlimiting since the process can readily be expanded to as many types ofspheres as desired. Moreover, the process can be used to build a desiredstructure and at a desired position insert a different type of spherelayer to achieve controlled layering of a third type of material at aparticular location. Many variations will occur to those skilled in theart upon consideration of the examples provided herein without departingfrom the present invention.

In addition to the process described above, a variation of the processcan be used (with or without the process above) to fabricate multi-layerphotonic crystalline structures. FIG. 8 is a flow chart 400 describingone such variation of this process starting at 402. At 406 a suitablesubstrate is prepared or obtained so that spheres of a particular typewill bond to the substrate in a manner dictated by the patterning ortemplating of the substrate as before. It is again hypothesized that asuitable bond can result from covalent bonding, electrostaticattraction, metallic bonding, hydrogen bonding, Van der Waals forces,hydrophobic/hydrophilic attractions, biological recognition such asprotein-protein/protein-ligand complexes (e.g., antigen-antibody), DNAor RNA hybridization or ligand-receptor (e.g., enzyme-substrate), orsome combination of the above forces, or any other suitable bond. Thespheres are then brought into contact with the prepared surface of thesubstrate at 410 by, for example, immersing the substrate in a solutioncontaining the spheres or exposing the prepared surface to such asolution or slurry containing the microspheres. At this point, dependingupon the materials and type of bonding, agitation, sonication, heatingor other actions may be taken to enhance the speed or consistency of thebonding of the spheres to the substrate. Once a self-passivated orotherwise self-limiting single layer of spheres have bonded to thesurface of the substrate, the excess spheres can be removed, for exampleby rinsing at 414. Alternatively, the substrate is exposed to a layer ofmicrospheres that weakly bond to it since a weak bond may beadvantageous ordering. After deposition, the excesss pheres are gentlyremoved and then the bond of the microspheres is strengthened oractivated by addition of additive chemicals such as glutaraldehyde, bychange in pH, by UV or other radiation exposure or any other mechanism.

If additional layers are desired at 418, the layer of spheres bonded tothe substrate are modified to permit bonding, either to the same kind ofsphere or to another kind of sphere at 422. The process of 410, 414, 418and 422 are then repeated until a crystal of desired size is achievedand the last layer is in place. The process is then halted at 426.

The process of FIG. 8 could be carried out, for example, using covalentbonding. Microspheres coated with phosphonate terminal groups have a2-charge, and would exhibit self-passivation. When introduced to asolution containing Cr⁴⁺ ions, these ions bond to the phosphonategroups, leaving them with a +2 charge. Subsequent exposure tophosphonated microspheres would add an additional layer to thecrystalline structure. Thus, if phosphonated spheres are used in process400, the spheres can be modified as in 422 to permit them to bond to oneanother by introduction of a solution containing Cr⁴⁺ ions. Additionallayers of phosphonated microspheres can then be built up to produce aphotonic crystal made of a single type of sphere.

The process of FIG. 8 could also be carried out, for example, usingbiological recognition. One example of this type of process would usemicrospheres coated with biotin, and modified with streptavidin. At 410a self-passivated layer of biotin-coated spheres is formed in a similarmanner as described previously. Excess spheres are then removed by, forexample, rinsing at 414. At 422, the spheres are exposed to a solutioncontaining streptavidin, which bonds to the biotin-coated surface of thespheres. Each streptavidin protein contains four sites for bonding, andso three of those sites are still available for subsequent bonding withbiotin-coated spheres after the initial bond. Additional layers ofbiotinylated microspheres can then be built up to produce a photoniccrystal made up of a single type of sphere.

Note that the processes in FIG. 7 and FIG. 8 refer only to the bondtypes, so that other attributes of the microspheres, such as size,shape, and/or chemical composition, may vary within or between thelayers. In addition, one could perform additional processing steps, forexample introduction of defects within a layer, before proceeding to addsubsequent layers to the crystal.

As previously described, it is believed that many types of bonds can beutilized (singly or in combination) to fabricate crystal structuresconsistent with certain embodiments of the present invention. Covalentcoupling is a bonding technique that might be used to selectivelydeposit monolayers of spheres. Covalent coupling has the advantage ofproviding the largest bond strength of the techniques mentioned. Inaddition to the phosphonate/Cr⁴⁺ linkage mentioned previously, numerouscovalent coupling chemistries are well known. They are often used tocovalently couple carboxyl- or amino-modified microspheres to moleculessuch as proteins which have terminal carboxyl and amino groups.Carboxyl- and amino-modified microspheres are available commerciallyfrom Bangs Labs, Inc (9025 Technology Drive, Fishers, Ind. 46038-2886)and other vendors. Several approaches for achieving covalent couplingbetween layers of microspheres in a photonic crystal are possible.Examples include linkages between carboxyl-modified microspheres andligands with available amines using a water soluble carbodiimide. Oneskilled in the art will appreciate that, several approaches can be takenusing this type of chemistry. One example would be to saturate a batchof carboxyl-modified microspheres with carbodiimide, and use this asmicrosphere type “A” in the process flow shown in FIG. 7. Thenamino-modified microspheres could be used as microsphere type “B”.

Another example of covalent coupling between microspheres using theprocess flow in FIG. 8 involves linkages between amino-modifiedmicrospheres using an amine-reactive homobifunctional cross-linker, suchas glutaraldehyde. The glutaraldehyde cross-linker bonds to amine groupson both ends of the molecule. Several approaches in applying this to theformation of photonic crystals are possible. For example, afterdepositing a layer of amino-modified microspheres, the layer could thenbe activated using a cross-linker, according to the process flow in FIG.8. A second approach would be to saturate a batch of amino-modifiedmicrospheres using the crosslinker and use this batch as type “A”microspheres, and then use the amino-modified microspheres without thecrosslinker as type “B” microspheres, and follow the process flow inFIG. 7.

Numerous other examples of covalent coupling reactions are described inthe literature involving, for example, hydroxyl, hydrazide, amide,chloromethyl, aldehyde, epoxy, and tosyl end groups, all of which areavailable commercially as functional groups on the surface ofmicrospheres (e.g., from Bang Labs and other sources). Another exampleutilizes techniques involving the creation of self-assembled monolayers(SAMs). Such techniques involve exposing a type of molecule, usually insolution, to a surface which bonds to one endgroup of the molecule. Theother endgroup may be functionalized for a variety of applications. Manyother covalent coupling chemistries will occur to those skilled in theart upon consideration of the present teaching.

As previously described, many types of bonds can be utilized tofabricate crystal structures consistent with embodiments of the presentinvention. Electrostatic charge is another bonding technique that mightbe used to selectively deposit monolayers of spheres. Electrostaticbonds occur between ionized groups of opposite charge (e.g., carboxyl(—COOH and amino (—NH₂)). It is preferred that spheres used in anelectrostatic bonding embodiment have an electrostatic charge that islocalized to the surface of the sphere. If the electrostatic charge iscentralized, as the charge is shifted away from the center of thesphere, the spheres might cluster and lose the self-passivation qualitydesired for the present invention. Several approaches are possible forcreating a charge localized to the surface of the sphere. Silica spheresnaturally have a negative surface charge due to the Si—H terminations atthe surface. Therefore the charge is inherently distributed on thesphere at the surface. Using this negative charge, it is relatively easyto coat the spheres leaving a positive charge, again distributed on thesurface. Other examples of microspheres with alternative surfacechemistries that exhibit electrostatic charges and are commerciallyavailable are polymeric microspheres with carboxyl surface groups, whichare negative, and amine surface groups, which are positive.Alternatively, the electrostatic charge could be generated in severalways such as ion implantation or plasma processing. Other surfacechemistries that exhibit electrostatic charge are consistent withcertain embodiments of the present invention.

Another example consistent with certain embodiments of the presentinvention, takes advantage of a common technique in biomaterialsresearch known as protein, RNA, DNA, or more generally, biologicalrecognition. A protein, RNA or DNA strand is bound to a surface, andwhen exposed to the antigen complementary RNA or DNA strand, or RNA orDNA binding protein the two compatible materials are strongly bound toeach other. This is normally detected by techniques such as fluorescenceor surface plasmon resonance spectroscopy. The same approach could beapplied to spheres, with alternating layers of spheres having surfacescomposed of protein and antigen, respectively for example. An advantageof this approach is the high selectivity provided by the proteins orDNA. A potential advantage of DNA or RNA coupling between microspheresis that the length of the strands can be selected over a wide range oflengths. This could allow tuning of the lattice parameters of thecrystal independent of sphere size. Also, the strands are elasticcompared to most molecules, which could allow easier tuning of thephotonic bandgap by mechanical stretching.

Using biological recognition, as described previously, a photoniccrystal could be formed by using alternating layers of biotin- andstreptavidin-coated spheres, respectively. Alternatively, a photoniccrystal could be formed using only one type of microsphere, in this casebiotin-coated microspheres, such that after each layer is deposited, themicrospheres are treated with a complemetary molecule, in this casestreptavidin, such that they are activated to bond with a subsequentlayer of microspheres.

Another use of biological recognition could involve one layerfunctionalized with a single strand DNA or RNA and the following layer acomplementary (antisense) oligonucleotide strand. Similarly, alternatinglayers providing antibody/antigen binding could be used, e.g. withprotein A and IgA, or protein G and IgG. Enzyme and substrate binding isnormally temporary, but this binding could be used between alternatinglayers by locking the molecules in an intermediate bound state by usingappropriate buffer conditions.

Another method for obtaining a self-passivated layer that appearsparticularly promising is the use of electrostatic charges in chargedpolymers such as polyelectrolytes. Polyelectrolytes are polymers withionizable groups on each monomer repeat unit. The deposition of a thinfilm layer can be achieved by adsorbing a polyelectrolyte of one chargefrom solution onto an immersed substrate, which may be planar, patternedor microspheres. Adsorption takes place with a simple dipping, but alsowith spraying, spin-coating and polymer stamping. Adhesive propertiesand tuning of the film characteristics can be achieved by multilayerdeposition. A substrate with a first polyelectrolyte film is capable ofadsorbing a second polyelectrolyte film of the opposite charge. Theprocess can be repeated alternating between the two polyelectrolytesolutions of opposite charge. This deposition is called layer-by-layerbut this terminology should not be confused with the layer-by-layermicrosphere deposition invented here. The polyelectrolyte multilayerproperties such as thickness and surface charge density can be tunedusing, among others, the number of layers, ionic strength, pH andsurfactants.

Polyelectrolytes can be of positive and negative charge and are called“weak” if the degree of ionization is pH dependent and “strong”otherwise. An example of a negative strong polyelectrolyte isPoly(sodium 4 styrenesulfonate) (PSS for short) [from Sigma-Aldrich,product number 527483], a positive strong polyelectrolyte is PDDA(Poly(diallyldimethylammonium chloride) [from Sigma-Aldrich, productnumber 409014], a positive weak polyelectrolyte is PAH Poly(allylamine)hydrochloride Aldrich 479144, ca. 65000]

The polyelectrolyte multilayers can be used as bonds to create thelayer-by-layer colloid crystals described herein. The bond between afirst layer of beads and the substrate can be fabricated by coating thesubstrate with a single- or multi-layer ending with a positive surfacecharge and using negatively charge microspheres such as carboxylatedpolystyrene microspheres, or by using microspheres with apolyelectrolyte coating ending with a negative surface charge. Bondsbetween microspheres can be achieved by using two kinds of microspheres,for example a 3 layer film of PDDA/PSS/PDDA creates positive surfacecharge on polystyrene microspheres that electrostatically bind tocarboxylated microspheres.

In another embodiment, the colloid crystal can be formed by alternatinglayers of spheres coated with an even-numbered polyelectrolytemultilayer and sphere coated with an odd-numbered polyelectrolytemultilayer.

In another embodiment, the microspheres deposited as first layer arecoated, after their adhesion on the substrate, with polyelectrolyte filmof opposite charge and become in this way an adhesive substrate for asecond layer of the same kind of spheres. Other variations will occur tothose skilled in the art upon consideration of the above teachings.

Many of the bond types described previously are typically produced insolution, but many, such as phosphonate/chromium bonds, can be driedafter bonding occurs. This means that no matrix, liquid or solid, isnecessary to bind the crystal together, although one could be used ifdesired or if otherwise beneficial. Removing the matrix material maysignificantly improve the index of refraction contrast between thespheres and surrounding medium (air), which is a major factor in theoptical behavior of the crystalline structure. A table of commonmaterials used as microspheres and/or matrix is given below. Removal ofthe matrix also improves the ability to introduce arbitrary liquid orvapor phase environments to the crystal so that it can more easily beused as a sensor. Removal of the liquid matrix would also allowinfiltration and densification of the volume between microspheres withsemiconductors or metals, which has been demonstrated using thetraditional colloidal approach. Subsequent removal of the microspheresis then possible.

Material Index of refraction Air 1.00 Water 1.33 Silica 1.46 Polystyrene1.59

There are several benefits to use of the methods described above. Forexample, with proper experimental conditions, defect levels are expectedto be much lower than for traditional colloidal techniques. Also,traditional colloidal techniques are limited to sphere sizes in the800-1000 nm range or smaller, because sedimentation effects becomeimportant for larger spheres. Because the current approach builds thecrystal a single layer at a time, sedimentation is not a concern. Thisshould allow larger spheres to be used so that crystals with opticalfeatures in the mid- and far-infrared can be fabricated. As mentionedpreviously, substrate templating and proper sphere size selection canallow more complex crystal structures than were previously possible. Forexample, non-close-packed structures such as body-centered cubic,tetragonal, and monoclinic structures can be designed and fabricated.Also, the crystals can be integrated into optical systems on a waferscale, and batch processing should make the process very cost-efficient.

Thus, in accordance with certain embodiments, the current inventionprovides several new techniques for the fabrication of photonic crystalscomposed of small particles, such as spheres. This layer-by-layerapproach as described herein allows one to tailor the properties of thecrystalline structure in ways not previously possible, which could notonly speed the development of commercially viable crystals, but allowthe design of structures with new functionality.

While the invention has been described in conjunction with specificembodiments, it is evident that many alternatives, modifications,permutations and variations will become apparent to those skilled in theart in light of the foregoing description. Accordingly, it is intendedthat the present invention embrace all such alternatives, permutations,modifications and variations as fall within the scope of the appendedclaims.

1. A method of fabricating a photonic crystal, comprising: providing asubstrate; exposing the substrate to a plurality of first microspheresmade of a first material, the first material being of a type that willbond to the substrate and form a self-passivated layer of firstmicrospheres to produce a first layer; and exposing the first layer to aplurality of second microspheres made of a second material, the secondmaterial being of a type that will bond to the first layer and form aself-passivated second layer of second microspheres.
 2. The methodaccording to claim 1, further comprising: exposing the second layer to aplurality of the first microspheres made of a the first material, thefirst material being of a type that will bond to the second layer andform a self-passivated layer of first microspheres.
 3. The methodaccording to claim 2, further comprising: repeatedly exposing a mostrecently formed layer to microspheres to a plurality of microspheresthat will bond to the most recently formed layer and self-passivate tofabricate a multiple layer photonic crystal.
 4. The method according toclaim 1, wherein the first microspheres comprise streptavidin-coatedmicrospheres and the second microspheres comprise biotin coatedmicrospheres.
 5. The method according to claim 4, wherein the substratehas biotinylated regions on a surface of the substrate.
 6. The methodaccording to claim 1, wherein the first microspheres comprisebiotin-coated microspheres and the second microspheres comprisestreptavidin-coated microspheres.
 7. The method according to claim 1,wherein the bond comprises at least one of covalent bonding,electrostatic attraction, metallic bonding, hydrogen bonding, Van derWaals forces, hydrophobic/hydrophilic attractions and biologicalrecognition.
 8. The method according to claim 1, wherein one of thefirst and second microspheres have DNA strands on a surface thereof, andwherein the other of the first and second microspheres have at least oneof complimentary DNA strands, complimentary RNA strands,oligonucleotides and DNA binding proteins on a surface thereof.
 9. Themethod according to claim 1, wherein one of the first and secondmicrospheres have RNA strands on a surface thereof, and wherein theother of the first and second microspheres have at least one ofcomplimentary DNA strands, complimentary RNA strands, oligonucleotidesand DNA binding proteins on a surface thereof.
 10. The method accordingto claim 1, wherein one of the first and second microspheres have aprotein situated on a surface thereof, and wherein the other of thefirst and second microspheres have at least one of an antigen and aligand that bonds to the protein on a surface thereof.
 11. The methodaccording to claim 1, wherein the first microspheres have a firstmolecule with a first endgroup on a surface thereof, and wherein thesecond microspheres have a second molecule with a second endgroup on asurface thereof, wherein the first and second molecules bond to eachother, but not to themselves, by formation of one of a covalent, ionic,metallic, hydrogen and Van der Waals bond.
 12. The method according toclaim 1, wherein one of the first and second microspheres have a bulkelectrostatic charge or a surface electrostatic charge of a first chargestate, and wherein the other of the first and second microspheres have asecond bulk electrostatic charge or surface electrostatic charge with asecond charge state which is opposite and attractive to the first chargestate, wherein the first and second microspheres bond to each other byformation of ionic/electrostatic bonds, but do not bond to themselves.13. The method according to claim 1, further comprising processing thefirst layer to form a surface that will bond to the second microspheresprior to exposing the first layer to the plurality of microspheres. 14.The method according to claim 1, wherein the substrate has a surfacecharge of a first polarity and wherein the first microspheres have acharge of a second polarity, and wherein the second microspheres have acharge of the first polarity.
 15. The method according to claim 1,wherein the first and second microspheres are coated with first andsecond polyelectrolyte layers, wherein the first and secondpolyelectrolyte layers have opposite charge.
 16. A method of fabricatinga photonic crystal, comprising: a) providing a substrate; b) exposingthe substrate to a plurality of first microspheres made of a firstmaterial, the first material being of a type that will bond to thesubstrate and form a self-passivated layer of first microspheres toproduce a layer of microspheres; c) modifying the first layer ofmicrospheres to permit the first layer of microspheres to bond withother microspheres to thereby produce a bondable layer; and d) exposingthe bondable layer to a plurality of second microspheres to form asecond layer of microspheres.
 17. The method according to claim 16,wherein the plurality of second microspheres are made of the firstmaterial.
 18. The method according to claim 16, wherein the plurality ofsecond microspheres are made of a second material.
 19. The methodaccording to claim 16, further comprising: modifying the second layer ofmicrospheres to permit the second layer of microspheres to bond withother microspheres and thereby produce a second bondable layer; exposingthe second bondable layer to a plurality of microspheres to form a thirdself-passivated layer of microspheres to produce a three layer photoniccrystal.
 20. The method according to claim 16, further comprisingrepeating c) and d) a plurality of times to achieve a desired number oflayers of a photonic crystal.
 21. The method according to claim 16,wherein the bond comprises at least one of covalent bonding,electrostatic attraction, metallic bonding, hydrogen bonding, Van derWaals forces, hydrophobic/hydrophilic attractions and biologicalrecognition.
 22. The method according to claim 16, further comprisingactivating the bond of the microspheres by at least one of thefollowing: addition of additive chemicals such as glutaraldehyde, bychange in pH, and by exposure to radiation.
 23. The method according toclaim 16, wherein the first microspheres have a first charge, andwherein the modifying comprises coating the first microspheres with apolyelectrolyte film having charge opposite the first charge.
 24. Themethod according to claim 23, wherein the second microspheres also havethe first charge.
 25. A photonic crystal structure, comprising: asubstrate processed to bond preferentially in selected areas; a firstlayer of first microspheres, the first layer being one microsphere deep,the first microspheres comprising a first material having a firstcoating and wherein the first coated microspheres are bonded to theselected areas of the substrate; a second layer of second microspheresone microsphere deep and bonded to the first layer of microspheres, thesecond microspheres having a second coating; and wherein the first andsecond coatings bond to each other but not to themselves.
 26. Theapparatus according to claim 25, wherein one of the first and secondmicrospheres comprise streptavidin-coated microspheres and the other ofthe first and second microspheres comprise biotin coated microspheres.27. The apparatus according to claim 25, wherein one of the first andsecond microspheres have RNA strands on a surface thereof, and whereinthe other of the first and second microspheres have at least one ofcomplimentary DNA strands, complimentary RNA strands, oligonucleotidesand RNA binding proteins on a surface thereof.
 28. The apparatusaccording to claim 25, wherein the one of the first and secondmicrospheres have DNA strands on a surface thereof, and wherein theother of the first and second microspheres have at least one ofcomplimentary DNA strands, complimentary RNA strands, oligonucleotidesand DNA binding proteins on a surface thereof.
 29. The apparatusaccording to claim 25, wherein one of the first and second microsphereshave a protein situated on a surface thereof, and wherein the other ofthe first and second microspheres have at least one of an antigen and aligand that bonds to the protein on a surface thereof.
 30. The apparatusaccording to claim 25, wherein first microspheres have a first moleculeon a surface thereof, and wherein the second microspheres have a secondmolecule on a surface thereof, wherein the first and second moleculesbond to each other but not to themselves.
 31. The apparatus according toclaim 25, wherein the substrate comprises an untemplated substrate. 32.The apparatus according to claim 25, wherein the bond comprises at leastone of covalent bonding, electrostatic attraction, metallic bonding,hydrogen bonding, Van der Waals forces, hydrophobic/hydrophilicattractions and biological recognition.
 33. The apparatus according toclaim 25, wherein the second microspheres are comprised of a secondmaterial.
 34. The apparatus according to claim 25, wherein the secondmicrospheres are comprised of the first material.
 35. The apparatusaccording to claim 25, wherein the substrate has a surface charge of afirst polarity and wherein the first coating on the first microsphereshave a charge of a second polarity, and wherein the second coating onthe second microspheres have a charge of the first polarity.
 36. Theapparatus according to claim 25, wherein the first and secondmicrospheres are coated with first and second polyelectrolyte layers,wherein the first and second polyelectrolyte layers have oppositecharge.
 37. A method of fabricating a photonic crystal, comprising:providing a substrate; providing a plurality of first microsohereshaving a first coating; bonding a single layer of the first microspheresone microsphere deep to the substrate to form a first layer; providing aplurality of second microspheres having a second coating, wherein thefirst and second coatings bond to each other but not to themselves; andbonding a single layer of second microspheres one microsphere deep tothe first layer to form a second layer.
 38. The method according toclaim 37, further comprising repeatedly bonding alternating layers offirst and second microspheres microspheres one microsphere deep toproduce a multiple layer photonic crystal.
 39. The method according toclaim 37, wherein the bond comprises at least one of covalent bonding,electrostatic attraction, metallic bonding, hydrogen bonding, Van derWaals forces, hydrophobic/hydrophilic attractions and biologicalrecognition.
 40. The method according to claim 37, wherein the substratecomprises an untemplated substrate.
 41. The method according to claim37, wherein alternating layers of the multiple layer photonic crystalare comprised of microspheres of differing types.
 42. The methodaccording to claim 37, wherein the substrate has a surface charge of afirst polarity and wherein the first microspheres have a charge of asecond polarity, and wherein the second microspheres have a charge ofthe first polarity.
 43. The method according to claim 37, wherein thefirst and second coatings comprise first and second polyelectrolytelayers, wherein the first and second polyelectrolyte layers haveopposite charge.
 44. A method of fabricating a photonic crystal,comprising: providing a templated substrate having a first charge; andexposing the templated substrate to a plurality of first microsphereshaving a polyelectrolyte coating carrying a second charge, the secondcharge being opposite the first charge so that the plurality of firstmicrospheres will bond to the templated substrate and form aself-passivated layer of first microspheres to produce a first layer.45. The method according to claim 44, further comprising: exposing thefirst layer to a plurality of second microspheres having apolyelectrolyte coating carrying the second charge in order to bond tothe first layer and form a self-passivated second layer of secondmicrospheres.
 46. The method according to claim 45, further comprising:exposing the second layer to a plurality of the first microsphereshaving a polyelectrolyte coating carrying the first charge in order tobond to the second layer and form a self-passivated layer of firstmicrospheres.
 47. The method according to claim 46, further comprising:repeatedly exposing a most recently formed layer to microspheres to aplurality of microspheres coated with a charged polyelectrolyte coatingthat will bond to the most recently formed layer and self-passivate tofabricate a multiple layer photonic crystal.
 48. The method according toclaim 47, wherein a last layer comprises carboxylated microspheres. 49.The method according to claim 45, wherein the first and secondmicrospheres are coated with one of Poly(sodium 4 styrenesulfonate) andPoly(diallyldimethylammonium chloride).
 50. A method of fabricating aphotonic crystal, comprising: a) providing a templated substrate; b)exposing the templated substrate to a plurality of first microspheresmade of a first material, the first material being of a type that willbond to the templated substrate and form a self-passivated layer offirst microspheres to produce a layer of microspheres; c) modifying thefirst layer of microspheres to permit the first layer of microspheres tobond with other microspheres to thereby produce a bondable layer bycoating the first microspheres with a polyelectrolyte film having afirst charge; and d) exposing the bondable layer to a plurality ofsecond microspheres having charge opposite the first charge to form asecond layer of microspheres.
 51. The method according to claim 50,further comprising: modifying the second layer of microspheres to permitthe second layer of microspheres to bond with other microspheres andthereby produce a second bondable layer by coating the second layer witha polyelectrolyte film; exposing the second bondable layer to aplurality of microspheres to form a third self-passivated layer ofmicrospheres to produce a three layer photonic crystal.
 52. The methodaccording to claim 51, further comprising repeating c) and d) aplurality of times to achieve a desired number of layers of a photoniccrystal.
 53. The method according to claim 50, wherein the first andsecond microspheres are coated with one of Poly(sodium 4styrenesulfonate) and Poly(diallyldimethylammonium chloride).
 54. Aphotonic crystal structure, comprising: a templated substrate processedto bond preferentially to a first material in selected areas; a firstlayer of first microspheres, the first layer being one microsphere deep,the first microspheres comprising the first material and bonded to theselected areas of the templated substrate; and a charged polymer coatingon the first microspheres.
 55. The apparatus according to claim 54,further comprising a second layer of second microspheres one microspheredeep and bonded to the first layer of microspheres, the secondmicrospheres having a charge that bonds to the charged polymer coating.56. The apparatus according to claim 54, wherein the charged polymercomprises a polyelectrolyte.
 57. The method according to claim 56,wherein the charged polymer comprises one of Poly(sodium 4styrenesulfonate) and Poly(diallyldimethylammonium chloride).
 58. Amethod of fabricating a photonic crystal, comprising: providing atemplated substrate; bonding a single layer of charged polymer coatedmicrospheres one microsphere deep to the templated substrate to form afirst layer; and bonding a single layer of charged polymer coatedmicrospheres one microsphere deep to the first layer to form a secondlayer.
 59. The method according to claim 58, further comprisingrepeatedly bonding a layer of charged polymer coated microspheres onemicrosphere deep to a most recently formed layer to produce a multiplelayer photonic crystal.
 60. The apparatus according to claim 58, whereinthe charged polymer comprises a polyelectrolyte.
 61. The methodaccording to claim 60, wherein the charged polymers are selected fromPoly(sodium 4 styrenesulfonate) and Poly(diallyldimethylammoniumchloride).
 62. A method of fabricating a photonic crystal, comprisingthe ordered steps of: a. providing a plurality of charged polymer coatedmicrospheres; b. bonding a single layer of the charged polymer coatedmicrospheres one microsphere deep to a substrate to form a first layer;and c. bonding a single layer of the charged polymer coated microspheresone microsphere deep to the first layer to form a second layer.
 63. Themethod according to claim 62, further comprising repeatedly bonding alayer of charged polymer coated microspheres one microsphere deep to amost recently formed layer to produce a multiple layer photonic crystal.64. The apparatus according to claim 62, wherein the charged polymercomprises a polyelectrolyte.
 65. The method according to claim 64,wherein the charged polymers are selected from Poly(sodium 4styrenesulfonate) and Poly(diallyldimethylammonium chloride).